The goal of this proposal is to develop methods to obtain high-resolution crystal structures of Family C G- protein-coupled receptors (GPCRs). These structures would greatly facilitate the development of effective and highly selective allosteric drug molecules for the treatment of psychiatric and neurological disorders. GPCRs are characterized by the presence of seven membrane-spanning alpha-helical segments separated by alternating intracellular and extracellular loop regions. GPCRs in vertebrates are commonly divided into five families by sequence and structural similarity, specified as Rhodopsin (Family A), Secretin (Family B), Glutamate (Family C), Adhesion, and Frizzled/Taste2. Family C GPCRs are structurally distinct from Family A receptors. While they share the same 7 TM topology, there is no sequence homology with Family A receptors. Family C receptors have a large extracellular, amino terminal ligand binding site consisting of a bilobed venus flytrap domain. However, it has been possible to modulate the activity of several Family C receptors by small molecule drugs that bind directly to the 7TM bundle and regulate receptor activity allosterically. The 7TM bundle is an ideal drug target because there is less sequence conservation between closely related receptor subtypes than observed for the native hormone binding site located in the Venus flytrap domain. Despite very active academic and industrial research efforts on drug discovery for GPCRs over the past two decades, the number of new GPCR drugs has been disappointing. One of the major bottlenecks in drug development has been the lack of high-resolution structural information on GPCRs for both identifying and optimizing leads. A recent advance in crystallization technology for family A GPCRs was developed in the laboratory of ConfometRx co-founder Brian Kobilka: generating GPCR-T4 Lysozyme (GPCR-T4L) fusion proteins. This technology has been applied to the high-resolution structures of six Family A GPCRs, opening new opportunities for structure-based design of drugs. We propose to adapt this technology to Family C receptors, using the metabotropic glutamate receptors (mGluRs) as a model system. The mGluRs are expressed primarily in the central nervous system and are potential therapeutic targets for the treatment of several neuropsychiatric disorders. In Phase I of this SBIR, we demonstrated the feasibility of using the GPCR-T4L technology to express and purify functional mGluR-T4L fusion proteins. In Phase II, we will proceed with the crystallization trials and structure determination and will use this information to initiate the development of a new class of mGluRs drugs. The methodology developed for generating mGluR-T4L proteins should be readily applicable to other Family C GPCRs. This proposal is in response to the NIH PA-10-081, Novel Tools for Investigating Brain-derived GPCRs in Mental Health Research, as well as Roadmap initiatives on the Structural Biology of Membrane Proteins.